Bloodworms would be a good addition to their diet, maybe even insect larvae too. The way I see it, goldfish are just ornamentally colored versions of their wild counterpart, a small species of Eurasian carp. That being said, Eurasian carp species (Commons, Crucians, Goldfish, etc) all share a very similar diet. While fishing for commons I've noticed that their natural diet consists of a pretty even proportion of meaty and green foods. The meaty intake being mainly terrestrial and aquatic insects, larger aquatic macroinvertobrates, and several species of crayfish. While the green part of their diet tends to be berries and nuts that fall in to the water from trees and sometimes a bit of algae. I almost exclusively spin fish for carp with corn and I fly fish for them with berry and terrestrial insect imitations. All that said, if I owned any of those carp species I'd try to keep their diet balanced pretty evenly between meaty and green foods.